1. Field of the Invention
Embodiments described in this specification relate generally to communications systems and more particularly to an analog baseband interface between wireless communication units.
2. Description of the Related Art
Wireless communications systems generally use radio frequency (RF) signals to transmit data from a transmitter to one or more receivers. Wireless communication systems are frequently used to implement wireless local area networks (LANs) in which data is transmitted and received between computers, servers, Ethernet switches, hubs, and the like. A wireless LAN may, for example, allow web page data to be transferred between a server and a computer.
Many wireless communication systems may be divided into two or more units. A typical division occurs between an RF unit and a baseband unit. The RF unit may convert transmit baseband analog signals into RF signals that may be transmitted through an antenna. The RF unit may also receive an RF signal from an antenna and convert the RF signal to a receive baseband analog signal. The baseband unit, working in conjunction with the RF unit, may create the transmit baseband analog signal the RF unit processes and transmits and may also receive a receive baseband analog signal from the RF unit that has been generated from a received RF signal.
The baseband unit is typically coupled to other units within the wireless communication system. Other typical elements of wireless communication systems may include elements configured to process data to be transmitted. For example, data may need to be encoded by an encoding element before the data can be processed by the baseband unit and then coupled to the RF unit for transmission. Still other typical elements of a wireless communication system may also include one or more digital signal processing units that can further process the data generated by the baseband unit from the analog signal received from the RF unit.
FIG. 1 illustrates an exemplary prior art wireless communication system 100 including a baseband unit 101 and an RF unit 111. Baseband unit 101 includes a digital to analog converter (DAC) 102 and an analog to digital converter (ADC) 103, whereas RF unit 111 includes a transmitter (TX) 112 and a receiver (RX) 113. One or more antennas 110 may be coupled to RF unit 111. In many cases, RX 113 and TX 112 may share antenna 110 (shown).
TX 112 of RF unit 111 is coupled via lines to DAC 102 in baseband unit 101. Many wireless communication systems are configured to transmit more than one RF signal contemporaneously. For example, two quadrature RF signals are usually transmitted to support orthogonal frequency-division multiplexing (OFDM) defined by wireless communication standards IEEE 802.11a or 802.11g. Therefore, TX 112 is usually configured to accept two transmit baseband analog signals. The two transmit baseband analog signals are also often relatively high bandwidth signals in order to support relatively high data transfer rates.
Differential line pairs are often used for high bandwidth signals in order to increase, among other things, noise immunity and performance. One embodiment of a differential line pair encodes a signal with a positive component and a negative component. These two components are typically implemented with two lines, each line carrying one component.
Oftentimes, the coupling between DAC 102 and TX 112 is through two differential line pairs. FIG. 1 shows two differential line pairs 107 (i.e. I+, I−, Q+, and Q−) from DAC 102 in baseband unit 101 to TX 112 in RF unit 111. RX 113 receives an RF signal through antenna 110 and recovers one or more receive baseband analog signals. As shown in FIG. 1, two differential line pairs also couple RX 113 to ADC 103 in baseband unit 101, thereby facilitating the contemporaneous receipt of two receive baseband analog signals (for the same reason as described above in the transmit case). Thus, wireless communication system 100 includes four differential line pairs coupling baseband unit 101 and RF unit 111 (i.e. eight discrete lines in total).
Multiple-input multiple-output (MIMO) wireless LAN architectures may provide improved performance when compared to single-input single-output architectures. The improved performance may be provided by, in part, using a plurality of transmitters and receivers (transceivers) to process RF signals. FIG. 2 illustrates a portion of an exemplary multiple transceiver wireless communication system 200, which can be characterized as an extension of the system configuration of FIG. 1 (but is not known to be implemented or discussed in the prior art). System 200, like system 100 (FIG. 1), includes a baseband unit 201 and an RF unit 211. However, in system 200, baseband unit 201 and RF unit 211 are divided into three sub-units, i.e. A, B, and C (indicated by the suffix of each reference number). Note that in other embodiments, baseband unit 201 and RF unit 211 may be divided into two sub-units or more than three sub-units.
A first baseband sub-unit 201A includes a first DAC 202A and a first ADC 203A, a second baseband sub-unit 201B includes a second DAC 202B and a second ADC 203B, and a third baseband sub-unit 201C includes a third DAC 202C and a third ADC 203C. A first RF sub-unit 211A includes a first transmitter (TX) 212A and a first receiver (RX) 213A, a second RF sub-unit 211B includes a second TX 212B and a second RX 213B, and a third RF sub-unit 211C includes a third TX 212C and a third RX 213C.
System 200, like system 100, uses differential line pairs to couple the elements in baseband unit 201 to the elements in RF unit 211. In system 200, two differential line pairs couple the DACs to the TXs and two differential line pairs couple the RXs to the ADCs. Therefore, to couple baseband unit 201 to RF unit 211, twenty-four discrete, inter-unit lines (i.e. lines between baseband unit 201 and RF unit 211) are required.
Note that wireless communication system 200 may be configured to enable loopback testing. Loopback testing is a testing method that allows a user to test or calibrate portions of a wireless communication system without the need to transmit or receive data to or from a second wireless communication system. Loopback testing, therefore, advantageously makes possible some amount of testing or calibration of the wireless communication system without relying on a separate wireless communication system.
Typically, during loopback testing, data passes through a loopback processing chain of elements that includes a DAC, a TX, a RX, and an ADC. Oftentimes, the loopback processing chain is configured such that the DAC is coupled to the TX that is coupled to the RX that is further coupled to the ADC. All the elements within the loopback processing chain may function contemporaneously to process test data. Specifically, the test data is often introduced into the loopback processing chain at the DAC, proceeds from the DAC to the TX, continues from the TX to the RX and finally travels to the ADC. The testing and calibration may come about by understanding the test data that is introduced to the loopback processing chain and examining the data that is returned from the loopback processing chain.
For example, using system 200 to test sub-unit A in loopback fashion, test data would be introduced to DAC 202A; DAC 202A would send data to TX 212A. The output of TX 212A would be sent to RX 213A. The output of RX 213A would then be sent to ADC 203A. The data from ADC 203A would then be examined. Thus, when wireless communication system 200 is configured in this fashion, the elements within the loopback processing chain may function contemporaneously, and one or more of the elements within the first baseband sub-unit 201A (i.e. DAC 202A and 203A) and the first RF sub-unit 211A (i.e. TX 212A and RX 213A) may be tested or calibrated.
One drawback to the architecture of system 200 is the relatively high inter-unit line count between baseband unit 201 and RF unit 211. Specifically, consider a typical implementation of wireless communication system 200 in which baseband unit 201 and RF unit 211 are on separate integrated circuits (ICs). In this implementation, each line coupling baseband unit 201 to RF unit 211 may require two pins, i.e. each IC may require one pin to connect to each line. Thus, each differential line pair may require four pins, i.e. each IC may require two pins to connect to each differential line pair. As a result, the twelve differential line pairs coupling baseband unit 201 and RF unit 211 may require twenty-four pins on each IC or forty-eight pins in total. As is well-known, relatively greater amounts of pins can significantly and undesirably increase the cost of an IC package.
Another drawback is that relatively large numbers of high-speed, differential line pairs, particularly differential traces used to couple baseband unit 201 and RF unit 211, may be relatively difficult to design. Specifically, differential traces may have relatively more stringent design rules than other, low speed traces. As is well-known, more stringent design rules generally require more design effort than less stringent design rules, such as those that may be required for low speed traces. Therefore, more differential lines pairs generally increase the design effort required to a design wireless communication system.
Therefore, a need arises to reduce the number of lines between baseband and RF units in a wireless communication system while still retaining the advantages of loopback testing.